Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed container to an external point outside the container. A conductive path is provided through the feedthrough by a conductor pin which is electrically insulated from the container. Many such feedthroughs are known in the art which provide the electrical path and seal the electrical container from its ambient environment. Such feedthroughs typically include a ferrule, the conductor pin or lead and a hermetic glass or ceramic seal which supports the pin within the ferrule. Such feedthroughs are typically used in electrical medical devices such as implantable pulse generators (IPGs). It has recently been discovered that such electrical devices can, under some circumstances, be susceptible to electromagnetic interference (EMI). At certain frequencies for example, EMI can inhibit pacing in an IPG. This problem has been addressed by incorporating a capacitor structure within the feedthrough ferrule, thus shunting any EMI at the entrance to the IPG for high frequencies. This has been accomplished with the aforementioned capacitor device by combining it with the feedthrough and incorporating it directly into the feedthrough ferrule. Typically, the capacitor electrically contacts the pin lead and the ferrule.
Some of the more popular materials employed to form the pin lead include tantalum and niobium. Unfortunately, tantalum and niobium are susceptible to oxide growth which can, depending on its extent, act as an insulator instead of a conductor over the surface of the pin lead. During fabrication of a feedthrough and capacitor combination, the pin is subjected to one or more heat treatments which can encourage oxidation, affecting the conductivity of the pin lead and its ability to make good electrical connections between other elements including the capacitor and so forth.
Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray or electromagnetic interference (EMI) signals for transmission to the interior of the medical device. In some prior art devices, ceramic chip capacitors are added to the internal electronics to filter and thus control the effects of such interference signals. This internal, so-called "on-board" filtering technique has potentially serious disadvantages due to intrinsic parasitic resonances of the chip capacitors and EMI radiation entering the interior of the device housing.
In another and normally preferred approach, a filter capacitor is combined directly with a terminal pin assembly to decouple interference signals to the housing of the medical device. In a typical construction, a coaxial feedthrough filter capacitor is connected to a feedthrough assembly to suppress and decouple undesired interference or noise transmission along a terminal pin.
So-called discoidal capacitors having two sets of electrode plates embedded in spaced relation within an insulative substrate or base typically form a ceramic monolith in such capacitors. One set of the electrode plates is electrically connected at an inner diameter surface of the discoidal structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing or case of the electronic instrument.
In operation, the discoidal capacitor permits passage of relatively low frequency electrical signals along the terminal pin, while shunting and shielding undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type are commonly employed in implantable pacemakers, defibrillators and the like, wherein a device housing is constructed from a conductive biocompatible metal such as titanium and is electrically coupled to the feedthrough filter capacitor. The filter capacitor and terminal pin assembly prevent interference signals from entering the interior of the device housing, where such interference signals might otherwise adversely affect a desired function such as pacing or defibrillating.
In the past, feedthrough filter capacitors for heart pacemakers and the like have typically been constructed by preassembly of the discoidal capacitor with a terminal pin subassembly which includes the conductive terminal pin and ferrule. More specifically, the terminal pin subassembly is prefabricated to include one or more conductive terminal pins supported within the conductive ferrule by means of a hermetically sealed insulator ring or bead. See, for example, the terminal pin subassemblies disclosed in U.S. Pat. Nos. 3,920,888, 4,152,540; 4,421,947; and 4,424,5511. The terminal pin subassembly thus defines a small annular space or gap disposed radially between the inner terminal pin and the outer ferrule. A small discoidal capacitor of appropriate size and shape is then installed into this annular space or gap, in conductive relation with the terminal pin and ferrule, by means of soldering, conductive adhesive, etc. The thus-constructed feedthrough capacitor assembly is then mounted within an opening in the pacemaker housing, with the conductive ferrule in electrical and hermetically sealed relation in respect of the housing, shield or container of the medical device.
Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the manufacture and installation of such filter capacitor assemblies has been relatively costly and difficult. For example, installation of the discoidal capacitor into the small annular space between the terminal pin and ferrule can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections. Moreover, installation of the capacitor at this location inherently limits the capacitor to a small size and thus also limits the capacitance thereof. Similarly, subsequent attachment of the conductive ferrule to the pacemaker housing, typically by welding or brazing processes or the like, can expose the fragile ceramic discoidal capacitor to temperature variations sufficient to create the risk of capacitor cracking and failure.
There exists, therefore, a significant need for improvements in feedthrough filter capacitor assemblies of the type used, for example, in implantable medical devices such as heart pacemakers and the like, wherein the filter capacitor is designed for relatively simplified and economical, yet highly reliable, installation. In addition, there exists a need for an improved feedthrough assembly having a discoidal capacitor which can be designed to provide a significantly increased capacitance for improved filtering. The present invention fulfills these needs and provides further advantages.
Disclosures relating to implantable medical devices, feedthroughs and capacitive filtering of EMI include the patents listed below in Table 1.